2 replies to this topic

[Butterfly]

Hello. I want to understand two things about flares. As far as I now, pilots are used to keep a small flame always on, so that the flared gas will burn when it is needed (for ex, at an emergency). Purge gas is used to keep a positive pressure in the flare header. Why? What could happen if there is no purge gas and a flow from an equipment is suddenly sent to flare? If the pilots are on, is purge gas burned in them? Or is it sent to atmosphere as a vent? Why dont we use purge gas as pilot?
Thank you

Edited by Butterfly, 28 August 2011 - 09:32 AM.

[kkala]

Role of flare pilot is as described in your post, i.e. to assure burning of flared gas under any conditions concerning flow rate or weather. A flare pilot has windshield; in case of flame failure an alarm is activated. Flare in critical services or strong wind can have more that one pilots, operating simultaneously.
Gas flow rate to flare varies from 0.0 to maximum flare capacity. When this flow is quite low (near 0), there is a risk of atmospheric air ingression into the header, through flare tip or even through valves (not being fully resistant to vacuum). Assume gas flow = 0, after a strong hot discharge to flare. As header and its content is progressively cooled, there will be negative pressure in header, thus air ingression into it. Even without mentioned gas contraction, air can enter the flare stack by diffusion.
Negative pressure / air ingression can be prevented by a small continuous gas (combustible or not) flow in the header, which is the purge gas flow. In previous exaple, this purge gas overbalances the contracted header volume, so that there is always some excess flow to atmosphere to prevent backflow. Water seal drum also prevents air from entering the header (but not the tip) (http://www.cheresour...h__1#entry50378).
Flare designs met (by me) were simple, where purge gas flow remained constant even during discharge. Sophisticated systems reducing purge gas during discharge, then increasing it , have been heard of but not met in design or field.
Purge gas flow is necessary in case of no other gas sent to flare for above safety reasons. If the purge gas is combustible, it is lighted by the pilots and burnt. Purge gas is introduced into the flare header, while pilot gas goes directly to pilot(s) at some pressures. Purge gas and pilot gas may have identical composition, but purge gas cannot be introduced to pilots after use, that is from header end; it is true however that flared gas reuse, as mentioned below, could realize it.
Flared gas reuse has been applied recently and info can be found in http://www.cheresou..._fromsearch__1 (especially in the web reference) indicating different purge gas needs between flare header and stack.

Edited by kkala, 29 August 2011 - 08:05 AM.

[ankur2061]

Butterfly,

You need to get hold of API STD 537 where the purpose and function of flare pilot and flare purge is very well defined. As per API 537 the purpose of flare pilot is as follows:

"The flare pilot must reliably ignite the fare. If the pilot fails, unburned hydrocarbons and/or toxic gases could be released to the atmosphere, potentially resulting in a vapor cloud explosion, odor problems or adverse health effects. In most elevated flare applications, the pilot cannot be accessed for service or replacement while the flare is in operation. The pilot system must be reliable enough to operate for years without maintenance."

For purge systems API 537 gives the following description:

"All flare systems are susceptible to flashback and explosion if not properly purged to keep air (oxygen) from entering the flare stack downward through the flare burner. To prevent air from entering the system during normal operation, a continuous purge is required. A purge conservation device may be installed in (or immediately below) the flare burner in order to reduce the amount of purge gas consumption.

Purge conservation devices are mechanical stationary designs. They reduce the use of purge gas while preventing some (but not all) air from entering downward into the flare stack. Several names are used to describe these seals, but some commonly used names are diffusion seals, velocity seals, venturi seals, and buoyancy seals."

Hope this helps.

Regards,

Ankur.

Edited by ankur2061, 29 August 2011 - 07:58 AM.